Exhaust gas aftertreatment device for internal combustion engines

ABSTRACT

An exhaust gas aftertreatment device for internal combustion engines having a catalyzer for the selective catalytic reduction of oxides of nitrogen from exhaust gases of motor vehicle diesel engines, provides overstoichiometric supply of NH 3  or materials releasing NH 3 . A first sensor records the NH 3  concentration contained in the exhaust gas and interrupts the supply of the NH 3  quantity when a specified upper threshold value is reached. A second sensor records the NH 3  adsorbed in the catalyzer, by way of which the NH 3  supply is resumed on reaching a specified lower threshold value. Alternatively, only one NH 3  sensor is provided in the exhaust gas aftertreatment device. The NH 3  concentration determined by this single sensor is compared, as the actual value, with a required value corresponding to a specified NH 3  concentration in order to form a correction signal which is used for triggering the metering appliance continuously connected into the gas phase.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to an exhaust gas aftertreatment devicefor internal combustion engines having a catalyzer for selectivecatalytic reduction of oxides of nitrogen from exhaust gases, especiallyfrom exhaust gases of motor vehicle diesel engines. More particularly,the present invention relates to a device comprising a meteringappliance for an overstoichiometric supply of one of NH₃ and materialsreleasing NH₃, at least two sensors, one of which is an NH₃ sensorconfigured to interrupt the supply when a quantity of NH₃ quantityexceeds a specified upper threshold value, and to resume the supplywhen, in the catalyzer, a stored NH₃ quantity reaches a specified lowerthreshold value, and also to a device comprising a metering appliancefor supply of one of NH₃ and of materials releasing NH₃, and a singlesensor for determining the NH₃ concentration in the exhaust gas.

As is well known, the oxides of nitrogen contained in the exhaust gasesare reduced to nitrogen and water on a catalyzer when a reducing agent,namely ammonia (NH₃) or compounds forming ammonia, is added.

Measures are disclosed in DE 3,825,206 to provide a pulsedoverstoichiometric metered addition of the reducing agent NH₃ and,specifically, by measuring the NO_(x) concentration before and after thecatalyzer, because the charge condition of the catalyzer is not definedin this mode of operation.

Furthermore, measures for the selective catalytic reduction of oxides ofnitrogen from exhaust gases are described in German Patent Application P41 17 143.8-43, by way of which the high NH₃ concentration occurring inthe metering phase is recorded by a sensor placed in the catalyzer. Thesensor interrupts the NH₃ supply after detection of the specified NH₃concentration. As soon as the NH₃ stored in the catalyzer issubstantially used up by the reaction, the renewed employment of the NH₃supply is determined by approximate calculation, from the enginecharacteristic diagram and the operating period, of the NO_(x) producedby the engine over the period since the beginning of metering or the endof metering, taking account of the average degree of separation.

An object of the present invention is to provide simple measures In anexhaust gas aftertreatment device provided for non-stationary internalcombustion engines to further improve the reduction of the oxides ofnitrogen contained in the exhaust gas.

The foregoing object has been achieved in accordance with the presentinvention by providing a further sensor as an NH₃ sensor recognizing thelower threshold value of the stored NH₃ quantity, or by supplying NH₃ inthe gas phase without metering pauses so that the NH₃ concentrationrecorded by the sensor is compared, as the actual value, with a requiredvalue corresponding to a specified NH₃ concentration to form acorrection signal usable for triggering the metering appliancecontinuously connected into the gas phase.

Due to the advantageous measures according to the present invention, itis not necessary to calculate the charge level in the catalyzer duringthe metering pause or NH₃ interruption phase on the basis of thecharacteristic diagram. The matching of the NH₃ supply to the variousengine types with very different exhaust gas emissions becomessuperfluous and it is not necessary to take account of the unavoidablecomponent scatter within a type in the selection of the level limits. Asecond ammonia (NH₃) sensor, which detects the ammonia adsorbed in thecatalyzer, undertakes the recognition of the lower charge level whereasthe first ammonia sensor detects gaseous ammonia.

This first NH₃ sensor can be placed either downstream of the catalyzeror in the catalyzer itself. Although the arrangement of the sensorwithin the catalyzer does not permit an optimum utilization of thecatalyzer volume, it does ensure that the NO_(x) emissions do not exceedthe permissible limiting values. The arrangement of the sensor after thecatalyzer permits, however, full utilization of the catalyzer volume forthe maximum adsorption capacity, but a brief minimum unallowable NH₃break-out cannot always be excluded.

Although a special embodiment with a second sensor is generallydescribed in the aforementioned German Patent application P 41 17143.8-43, this special configuration is subjected to exhaust gasupstream of the catalyzer in contrast to the present invention.

Continuous control of a slight, but constant NH₃ slip within permissiblelimiting values is possible by measures in which the NH₃ is supplied inthe gas phase without metering pauses so that the NH₃ concentrationrecorded by the sensor is compared, as the actual value, with a requiredvalue corresponding to a specified NH₃ concentration to form acorrection signal usable for triggering the metering appliancecontinuously connected into the gas phase. The advantage of thisarrangement lies in avoiding the need for any type of characteristicdiagram and in the compensation for any changes to the engine and thecatalyzer within the control range.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects, features and advantages of the presentinvention will become more readily apparent from the following detaileddescription thereof when taken in conjunction with the accompanying solefigure which is a schematic diagram of an internal combustion engine andcatalyzer utilizing the present invention.

DETAILED DESCRIPTION OF THE DRAWING

An internal combustion engine is designated generally by numeral 1, anexhaust gas conduit by numeral 2, a catalyzer by numeral 3 and ametering appliance by numeral 4. The metering appliance 4 comprises areducing agent tank 5 and a supply conduit 6 with a delivery pump 7 anda shut-off valve 8. The supply conduit 6 opens into the exhaust gasconduit 2 upstream of the catalyzer 3. The system also includes an airfilter 14 and a compressed air valve 15. The reducing agent tank 5contains ammonia (NH₃) or materials releasing ammonia, which aresupplied in a controlled manner to the exhaust gas flow in the exhaustgas conduit 2.

The catalyzer 3 is arranged in a casing 9 in which a first NH₃ sensor 10is provided in one embodiment downstream of the catalyzer 3. This NH₃sensor 10 measures the NH₃ concentration in the exhaust gas and feeds aswitching signal into a control unit 11 at a time when the gaseous NH₃quantity has reached a fixed upper threshold value. The control unit 11controls the delivery pump 7 in the sense of switching it off so thatthe NH₃ supply is interrupted.

A second NH₃ sensor 12 is arranged in, for example, carrier material ofthe catalyzer 3 to detect adsorbed NH₃. As soon as the lower level limitis reached in the catalyzer 3 and the NH₃ stored in the catalyzer hasbeen substantially consumed by reaction, a switch signal correspondingto the fixed lower NH₃ threshold value is supplied to the control unit11. The control unit 11 controls the delivery pump 7 in the sense ofswitching it on again and metered addition of NH₃ is resumed and, infact, as a function of operating parameters such as engine rotationalspeed n, control distance CD, exhaust gas temperature T_(exhaust)upstream of the NH₃ supply and exhaust gas temperatures at inlet,T_(cat).in and outlet, T_(cat).out of the catalyzer 3. During themetering pause, the shut-off valve 8 triggered by the control unit 11shuts off the supply conduit 6, into which no exhaust gas can flow.

The first NH₃ sensor 10 can, however, also be placed in the catalyzer 3as indicated by the numeral 10' to measure, in contrast to the secondNH₃ sensor 12 which measures lower charge level, gaseous NH₃.

A further embodiment of the present invention provides for continuouscontrol of the metering appliance 4 by the arrangement of a single NH₃sensor 13 downstream of the catalyzer 3. It is also possible to placethis NH₃ sensor 13 in the catalyzer 3 itself. The sensor 13 determinesthe respective NH₃ concentration in the gas phase. The instantaneouslypresent NH₃ concentration is compared, as the actual value, with arequired value corresponding to a specified NH₃ concentration and acorrection signal formed from this is used for triggering the meteringappliance 4. The continuous control provides an ammonia slip, within thepermissible limiting values, which is as small as possible and constant.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

We claim:
 1. An exhaust gas aftertreatment device for an internalcombustion engine having a catalyzer for selective catalytic reductionof oxides of nitrogen from exhaust gas, comprising a metering appliancefor an overstoichiometric supply of one of NH₃ and materials releasingNH₃, and at least one NH₃ sensor configured to interrupt the supply whenan NH₃ quantity exceeds a specified upper threshold value, and an NH₃sensor configured to recognize a specific lower threshold value of astored NH₃ quantity and to resume the supply when, in the catalyzer, thestored NH₃ quantity reaches the specified lower threshold value.
 2. Thedevice according to claim 1, where the exhaust gas is an diesel engineexhaust gas.
 3. The device according to claim 1, wherein the first NH₃sensor and the another NH₃ sensor are operatively arranged in thecatalyzer such that the first NH₃ sensor measures the NH₃ concentrationin the exhaust gas and the another NH₃ sensor measures the NH₃ adsorbedin the catalyzer.
 4. The device according to claim 1, wherein theanother NH₃ sensor is operatively arranged in the catalyzer and thefirst NH₃ sensor is operatively arranged downstream of the catalyzersuch that the first NH₃ sensor measures the NH₃ concentration in theexhaust gas and the another NH₃ sensor measures the NH₃ adsorbed in thecatalyzer.